Special Series: Flexing Mussels
The St. Croix River is one of the best mussel (aka freshwater clams) laboratories in America, because there is an incredible number of species present, and because their populations are typically large.
This summer, I joined three separate teams of scientists doing mussel research on the river. The adventure began with swimming in the St. Croix searching for Wabash pigtoe. Then looking for spectaclecase with the Wisconsin DNR. This final installment features one of the rarest and most endangered creatures on the continent.
“The uncommon richness of mussel species in the St. Croix parallels the uncommon richness of the flora and fauna of the watershed as a whole.”– U.S. Fish & Wildlife Service
Many mussel species almost should not exist. Their lives are so precariously dependent on such specific conditions that their continued ability to not only survive but reproduce, generation after generation, is remarkable.
Even minor disruptions to their homes can cause the whole thing to fall apart. Pollution can poison them. They might lose access to a fish species they can trick into carrying mussel babies for a first stage of life, quickly ending any chance of reproduction. Subtle changes in water chemistry, like the ratio of nutrients or ammonia or any number of other components, can weaken the creatures, especially by killing young mussels. Increased sediment can smother them. Climate change can disrupt critical ecosystem connections.
The threats are abundant, but somehow these species have not just survived over the ages, but become successful specialists. It’s why the St. Croix with its 41 species is special, being able to still support such diverse needs.
The winged mapleleaf mussel (Quadrula fragosa) has come about as close to extinction as possible, but it too hangs on here. It was once found throughout the Mississippi River and its tributaries, including the nearby Minnesota and upper Mississippi Rivers. All told, winged mapleleafs once inhabited at least 34 river systems in 12 states.
Now, the entire population has been reduced to four streams: the Ouachita and Saline Rivers in Arkansas, the Bourbeuse River in Missouri, and a small stretch of its former range in the St. Croix, which is the only place where it is known to reproduce.
University of Minnesota mussel researcher Mark Hove told St. Croix 360 that 200 years ago, winged mapleleaf were probably found the entire length of the St. Croix. He and others have collected old shells of the species from Wild River State Park to Stillwater. But today, living winged mapleleafs are found from Interstate State Park to William O’Brien State Park, with the majority in just the few miles between Interstate and Franconia.
Their future is perilous.
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Winged mapleleaf have the highest level of protection possible in the federal Endangered Species Act, and is listed as endangered by both Minnesota and Wisconsin. The International Union for Conservation of Nature and Natural Resources classifies it “Critically Endangered.” Their current range in the St. Croix is protected by the National Park Service.
Ensuring they don’t go from endangered to extinct is a priority. A collaborative effort to help the species includes the U.S. Fish and Wildlife Service, the National Park Service, and the Minnesota Department of Natural Resources.
Which is why a team of federal biologists donned wetsuits and stepped out of a boat into the St. Croix River one morning in September. Megan Bradley and her colleagues were checking in on a community of winged mapleleaf mothers.
The timing of the mussel development is tied to conditions around them, particularly water temperatures in late summer. This also affects the movements and behavior of their host fish, the channel catfish, and so the whole cycle does a delicate dance depending on a fragile rhythm.
At this site near the Minnesota Interstate Park boat landing, in sight of many thousands of people each year, but hidden by just a few feet of water, a creature was preparing to try something that is not believed to happen anywhere else on Earth: winged mapleleafs would propagate a new generation.
The U.S. Fish and Wildlife Service is trying to help that happen. The agency’s scientists are going so far as to help them reproduce successfully by providing a controlled environment.
The crew clad in wetsuits and snorkels wading in the strong current was part of a careful effort to raise the young in a laboratory. Someday, these young could help establish new self-sustaining populations in other rivers where winged mapleleaf have been lost, or in other parts of the St. Croix.
It has been tricky.
“In the wild the larval mussels remain attached to the catfish until late spring when the water begins to warm,” the agency reports. “Hatchery staff have worked hard to replicate this experience for the catfish and their mussel riders in the lab, but it has proven to be challenging.”
First, they must be closely monitored. Male mussels eject sperm into the water, where it reaches females and fertilizes their eggs, which then grow for a short time in their mother’s embrace.
This is when they must be collected and carried back to the lab. Too early and they won’t be fertilized, or won’t be mature, but too late and the eggs will already be fending for themselves, hopefully having found their way to a channel catfish.
Like most mussels, winged mapleleaf require specific fish species to carry their young for a while. For a long time, it was thought that only blue catfish would satisfy winged mapleleaf — a fish species that is no longer found in the St. Croix River. Then scientists discovered the mussels were also using channel catfish, which are still around.
In the end of the early September outing, only a few mussels were ready to go back to the lab. The biologists would be back many more times in the weeks ahead to monitor their progress and carry them back to a maternity ward they’ve prepared some 180 miles away.
Thanks to that previous research, the USFWS has developed a system to support reproduction at a lab located in the federal Genoa Fish Hatchery along the Mississippi Great River Road in southwest Wisconsin.
The females are brought to the lab when their larvae are ready, and then they are placed in a tank with channel catfish. Once they have infested the fish with the larvae, the mother mussels are brought back to the river. The catfish live in an aquarium designed to mimic the natural seasonal cycles of water temperatures. When they drop the mature larvae off their gills, the scientists collect the juveniles and then raise them in individual petri dishes.
It has been slow going. The channel catfish host species was only discovered in 2005. The first mussels were successfully propagated in 2016. Somehow the wild and untamed St. Croix River has so far more successfully supported reproduction than some very smart scientists.
Winged mapleleaf are just made to live and reproduce in rivers like the St. Croix.
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